R8_p1 Historical Earthquakes in the Himalaya Also see Stacey - PDF document

http://www.geo.tv/quake/kashmir.asp Kashmir Earthquake: The Mw7.6 Kashmir 8 October 2005 earthquake occurred in a region where a great plate-boundary earthquake has long been considered overdue. Although the earthquake resulted in widespread devastation, it is doubful that it has released more than one tenth of the cumulative elastic energy that has developed since the previous great earthquake in the region in 1555 or earlier. This overview places the earthquake in a historical and structural context. The figures below, published in May 2001 and in May 2005 respectively, illustrate the slip potential developed along the arc (left: red=certain, pink=possible) and the possible size and locations of earthquakes in these regions based on recent findings (right). The area of each yellow region is proportional to estimates of potential elastic energy to be released (Bilham and Wallace, 2005). The Kashmir earthquake overlapped the extreme left-hand rectangle. Click for larger image with the Kashmir earthquake superimposed. R8_p1

Historical Earthquakes in the Himalaya Also see Stacey Martin's ASC page. previous catastrophic earthquake in Kashmir occurred in 1555 but we have insufficient data to assign it a magnitude. Five decades earlier a moderate Mw ≈ 7.3 earthquake damaged Kabul, and a great 600-km-long 8.2<Mw<8.6 rupture in the central Himalaya destroyed monasteries in Tibet and buildings in Agra. Moderate earthquakes similar to the 2005 Kashmir event occurred in 1885 (Mw=7.5), 1905 (Mw7.8 Kangra), in 1842 (Mw7.5 Kunnar), and in 1974 (Mw=7.4 Pattan). Intensity distribution R8_p2

Using internet responses and reports in newspapers, a map of the decay in shaking intensities as a function of distance from the epicenter has been developed by Stacey Martin at the ASC. The preliminary version of this map shown here illustrates the long reach of the earthquake ranging from Intensity 12 near the epicenter (red=total destruction), to barely felt (violet), beyond distances of hundreds of km. Click for original. Compare with Susan Hough's intensity distribution for the 1905 Mw=7.8 earthquake. How bad was it? If reports from the Kashmir epicentral region are confirmed, the number of fatalities exceeds 40,000 making it the most fatal earthquake ever to occur in the Indian subcontinent . The number of fatalities in an earthquake is linked to the vulnerability of local buildings, population density, and shaking intensity. In 1935 a Richter magnitude M7.5 strike-slip earthquake near the city of Quetta, the only large settlement in an otherwise sparsely populated region of Afghanistan, Pakistan and Baluchistan, resulted in an estimated 35,000 dead. The M7.8 Kangra earthquake in 1905 caused 20,000 fatalities, and the Mw=7.6 Bhuj 2001 earthquake 18,500. R8_p3

Deaths vs earthquake magnitude for earthquakes throughout the world 1900-2004 compared to the Kashmir 2005 earthquake. Although a simple relation between earthquake magnitude and the number of resulting deaths can be discerned (gray shading), the fatal consequences of large earthquakes depends more on their proximity to urban populations, the vulnerability of dwellings, and the time of day, than on the energy released. (fig. from Hough & Bilham, 2005). The Sumatra/Andaman/Nicobar earthquake resulted in epicentral building collapse and damage, and huge loss of life along a thin but concentrated population on a coastal strip throughout the Indian ocean and Andaman sea. Is this a foreshock to a larger quake? Probably not, but we have so few well- documented examples of Himalayan earthquakes that it is conceivable that another earthquake could occur. The damaging Kashmir 1555 earthquake was preceded in 1501 by a damaging earthquake of unknown magnitude also in Kashmir. Also it is known that Himalayan earthquakes can trigger others. For example in 1833, two major earthquakes preceded the Mw=7.8 mainshock in Nepal by 5 hours and 15 minutes respectively, alerting local residents and resulting in a very minor loss of life. The 1909 Afghan earthquake consisted of two events 1 minute apart, and the 1905 Kangra earthquake had a M7 earthquake embedded in its coda some 7 minutes after the mainshock. We know of no great earthquake triggered days to months after a major earthquake in the same region. However, speculation initiated by the recent R8_p4

Sumatra earthquake that the rupture zones of even quite large earthquakes can re- rupture, suggests that recent M ≤ 7.9 ruptures cannot be considered immune to premature failure in a larger earthquake (Bilham and Wallace, 2005, Feldl and Bilham, 2005). Landslides in the region may result in floods in the next few months. This Aster image processed by Eric Fielding shows the 3 km path of a landslide near the epicenter that has over-ridden a facing spur and dammed two tributaries of the Jhelum river top right. Lakes will grow upstream (to the left and below) until the dam is breached. A catastrophic downstream flood may occur should the dam not breach early in reservoir development. Tectonic setting Earthquakes occur throughout the Indian subcontinent but most are located along the plate boundaries to west, north and east. The 1935 Mw=7.5 Quetta earthquake occurred on part of the left-lateral Chaman fault system on the western boundary. The 2004 Mw=9.3 Sumatra/Andaman earthquake ruptured the right lateral eastern plate boundary. In contrast, the Kashmir 2005 earthquake occurred near the western end of the Himalaya. Thrust earthquakes here signify the descent of India beneath Tibet. The structure on which the mainshock occurred - the Hazara syntaxis - is known to be among the most active in the Himalaya, with local uplift rates in the past several thousand years of ≈ 1 cm/yr. R8_p5

Earthquake details Teleseismic data indicate that a 100x50 km wide plane oriented N27W slipped unevenly 1-6 m with most slip 2/3 from the NW end. Chen Ji at Caltech using the USGS mainshock location of 34.432N & 73.537E indicates a preferred nodal plane dipping 37deg NE with dimensions 90-100 km along strike by 35-50 km down-dip. The moment release is 2.1x10^27 dyne.cm ( Harvard 2.7x10^27 dyne.cm). The mean slip derived from the geometric moment for Mw7.6 assuming a 100 km long, 50 km wide rupture is roughly 2 m. The Harvard CMT indicates 1.4 m. Using these preliminary values we have calculated approximate surface strain-fields and displacement fields (below). The surface dilation calculated from Coulomb 2 (Toda et al.) is shown in map view and in profile. Red areas have been stretched and violet regions squeezed by 1 part in 100,000 by the earthquake. GPS Points in the area The right hand map (click for larger version) shows aftershocks to 12 Oct with Chen Ji's slip solution for the rupture and principal faults (top right Karakorum fault). It is probable that some relocation of the aftershocks and rupture area will occur as improved models are calculated. In particular the mainshock/aftershock geometry is parallel to the projected strike of the Himalaya, whereas the synthetic rupture and mainshock strike is skewed 20 deg clockwise parallel to the Hazara syntaxis. Points of a sparse GPS network measured once collaboratively with the University of Peshawar are located South, West and NE of the rupture (red triangles). New points are being re-occupied to the east of the rupture zone by Indian geodesists in a search for afterslip. Synthetic models for R8_p6

convergence suggest that afterslip will be pronounced in the 100 km NE of the rupture. References 2000-2005 Ambraseys, N., and R. Bilham, A note on the Kangra Ms=7.8 earthquake of 4 April 1905. Current Scienc e, 79 , 101-106, 2000. Ambraseys, N., and R. Bilham (2003), Earthquakes in Afghanistan, Seism. Res. Lett., 74(2), 107-123. Electronic supplement Ambraseys, N., and R. Bilham, MSK Isoseismal intensities evaluated for the 1897 Great Assam Earthquake, Bull. Seism Soc. Am.93 (2) 655-673, 2003 Bilham R and K Wallace, (2005), Future Mw>8 earthquakes in the Himalaya: implications from the 26 Dec 2004 Mw=9.0 earthquake on India's eastern plate margin, Geol. Surv. India Spl. Pub. 85, 1-14. Bilham, R. (2005) A flying start followed by slow slip. Science, 308(5725),1126. Bilham, R. and P. England, Plateau pop-up during the great 1897 Assam earthquake. Nature,410, 806 - 809 (2001) Bilham, R., V. K. Gaur and P. Molnar, Himalayan Seismic Hazard, Science , 293 , 1442-4, 2001. Bilham, R., and N. Ambraseys, (2004) Apparent Himalayan slip deficit from the summation of seismic moments for Himalayan earthquakes, 1500-2000, Current Science, 88(10), 1658-1663, 2005. Bilham, R., E. R. Engdahl, N. Feldl and S. P. Satyabala. 2005 Partial and Complete Rupture of the Indo-Andaman plate boundary 1847-2004, Seism Res. Lett, 76(3), June 2005. Bilham, R., R. Bendick, and K. Wallace, (2003). Flexure of the Indian Plate and intraplate earthquakes, Proc. Indian Acad. Sci. (Earth Planet Sci.),112(3) 1-14 Bilham, R., Slow tilt reversal of the Lesser Himalaya between 1862 and 1992 at 78°E, and bounds to the southeast rupture of the 1905 Kangra earthquake, Geophys. J. Int (2001) 144, 1-23. Feldl N & R. Bilham, Great Himalayan Earthquakes and the Tibetan Plateau, submitted to Nature 2005. Hough S. E., R. Bilham, N. Ambraseys, and N. Feldl, Revisiting the 1897 Shillong and 1905 Kangra earthquakes in northern India: Site Response, Moho reflections and a Triggered Earthquake, Current Science, 88(10), 1632-8, 2005. Hough, S and R. Bilham, (2003) Shaken to the Core, Natural History Magazine, 112(1),42-48. R8_p7